We analyzed the largest clinical experience to date with Anthrax Immune Globulin Intravenous (AIG-IV) from an outbreak of Bacillus anthracis soft tissue infection in injection drug users and compared findings from 15 AIG-IV recipients versus 28 non-recipients. Overall, mortality rates did not differ comparing recipients vs. non-recipients (33.3% vs. 21.4% respectively, p=0.47). However, whereas only 8 of 30 patients (26.7%) with a low risk of death admission sequential organ failure assessment score (SOFA) of 0 to 5 received AIG-IV (p=0.01compared to a proportion of 50%), 7 of the 13 patients (53.9%) with a high risk of death (SOFA of 6 to 11) received treatment (p=0.78). Overall, AIG-IV recipients had surgery more often and, among survivors, longer hospital stays (all p0.05). Overall, AIG-IV recipients were sicker than non-recipients in this outbreak. This difference and the small number of higher risk patients, confound assessment of AIG-IVs effectiveness in the outbreak.